The present invention relates to a perfected electromagnetic metering valve for a fuel injector, in particular for internal combustion engines.
The metering valves of fuel injectors normally comprise a control chamber with a discharge conduit, which is normally closed by a shutter by means of a main spring, and which is opened by energizing an electromagnet to so move the armature as to overcome the force exerted by the spring. In known valves, the armature is normally rigidly connected to a stem sliding inside a fixed guide.
When closing the discharge conduit, the kinetic energy of the armature and the stem is dissipate in the impact of the shutter against the valve; and, when opening the discharge conduit, the kinetic energy of the return stroke of the armature and the stem is dissipated in the impact of the stem against a stop.
Such impact generates considerable force proportional to the mass and speed of the armature and stem and inversely proportional to the duration of impact, which is very short; and, on account of the hardness of the stem, ball and body of the valve, results in considerable rebound, both when opening and closing the valve, so that the movement of the armature fails to provide for steady operation of the injector.
One proposal to reduce rebound of the mass arrested in both the opening and closing stroke is to disconnect the armature from the stem, and provide a second spring weaker than the main spring and for pushing the armature against an element of the stem. In yet another known valve, the stem is provided with a flange housed inside a chamber in which fuel is circulated, and in which the movement of the flange creates a certain amount of turbulence to further reduce rebound.
Such known valves, however, present the drawback of not allowing a small interval between two consecutive movements of the armature, as requested, for example, by high-speed injection engines. In particular, such valves are unsuitable for engines requiring a preinjection of fuel shortly before the main injection. In which case, in fact, the overtravel of the armature with respect to the travel of the stem prevents the armature from returning to the idle position prior to the main injection.